The present invention relates to an R-TM-B permanent magnet member having an improved corrosion resistance.
Due to the recent trend of miniaturization of electric and electronic appliances having higher performance, permanent magnet members, parts of such appliances, are also demanded to have higher performance and smaller sizes. Conventionally most powerful permanent magnets are rare earth metal-cobalt (R-Co) permanent magnets, but a more powerful R-TM-B permanent magnet has been developed (U.S. Pat. No. 4,864,406). In the above R-TM-B magnet, R represents one or more rare earth metals including Y, TM represents a transition metal based on Fe or Co, part of which may be substituted by other metals or non-metals, and B represents boron.
However, since the R-TM-B permanent magnets are easily rusted, their surfaces are provided with anti-oxidizing coating layers to improve their corrosion resistance. The types of the anti-oxidizing coating layers proposed so far include a nickel plating layer, an anti-oxidizing resin coating layer, an Al ion plating layer, etc. Among them, the nickel plating layer has been paid much attention as an effective means for easily improving the corrosion resistance of the R-TM-B permanent magnets (Japanese Patent Laid-Open No. 60-54406). The nickel plating layer is more advantageous than the anti-oxidizing resin coating layer in that the former shows higher mechanical strength and smaller moisture absorption.
Nevertheless, unlike the anti-oxidizing resin coating layer, the nickel plating layer disadvantageously has pinholes. Therefore, despite its low moisture absorption, moisture permeates the nickel plating layer to a surface of the permanent magnet through pinholes of the nickel plating layer as the time elapses, causing corrosion of the permanent magnet.
To solve this problem, various attempts have been proposed: A two-layer Ni plating method, a method of coating a nickel plating layer with an anti-oxidizing resin to cover pinholes (Japanese Patent Laid-Open No. 63-110707).
However, in the above methods, the adhesion of a lower nickel plating layer to an upper nickel plating layer or to an anti-oxidizing resin coating layer, and the corrosion resistance of the upper nickel plating layer and the anti-oxidizing resin coating layer are not sufficient. Accordingly, these methods fail to provide a high corrosion resistance to the R-TM-B permanent magnet members.
In addition, the plated R-TM-B permanent magnet members are likely to be covered with a thin organic film, which prevents the R-TM-B permanent magnet members from being well adhered to substrates. Accordingly, in order to bond the R-TM-B permanent magnet members to the substrates of electric or electronic appliances strongly, it is important to improve the adhesion properties of the R-TM-B permanent magnet members.